1. Field of the Invention
This invention relates to an improved mine roof anchor assembly with a friction reducing means and more particularly to a method and apparatus for reducing the torque required to exert a preselected lateral force of the expansion leaves or fingers against a mine roof bore hole wall.
2. Description of the Prior Art
There are several major problems in the known expansion shell assemblies, both the bail type assemblies with a pair of expansion leaves and the mechanical expansion shells having a plurality, usually four, expansion fingers connected to and extending upwardly from a base ring, that cause operating difficulties in anchoring a bolt under tension in a bore hole in a mine roof.
One of the problems is the excessive amount of torque that must be applied to the bolt to expand the leaves or fingers and apply a preselected force on the leaves against the bore hole wall to properly and adequately engage the serrated surfaces of the leaves or fingers to the bore hole wall. The expansion leaves or fingers are urged laterally against the bore hole wall by the sloped surfaces of the plug as the bolt is rotated to move the plug downwardly relative to the expansion leaves or fingers. The plug sloped surfaces and the expansion finger surfaces are in abutting relation and the plug sloped surface moves relative to the inner surface of the leaves or fingers. Both the plug and the expansion leaves or expansion shells are usually formed by sand casting or the leaves may be fabricated from a soft steel. In either case, the surfaces are rough and irregular with a high coefficient of friction between these surfaces. As the leaves or fingers are urged outwardly against the bore hole wall, the friction between the surfaces increase and an excessive torque must be applied to the bolt to move the plug downwardly and provide the required lateral force on the leaves or fingers to anchor the serrated portions of the expansion shell against the bore hole wall to maintain the desired anchorage and tension on the roof bolt anchored in the bore hole.
There is a need for an expansion shell assembly that requires less torque to be applied to the bolt to obtain the desired lateral force on the expansion leaves or fingers to mechanically anchor the bolt assembly in the bore hole.
Another problem of certain mechanical anchors is a "spinner" where upon rotation of the bolt in the bore hole, the expansion leaves or fingers fail to expand or move laterally a sufficient distance to engage the wall of the bore hole and the expansion leaves or expansion fingers continue to rotate with the bolt so that the plug does not move downwardly on the bolt to expand the expansion leaves into engagement with the bore hole wall.
It is believed a major cause of "spinners" is that the friction between the abutting surfaces of the plug and leaves or fingers is greater than the friction between the outer surface of the leaves and the bore hole wall. External factors such as wet bore holes, soft roof strata and fluid like resin in the bore hole reduce the friction between the bore hole wall and the outer surface of the expansion leaves or fingers and contribute to the "spinner" problem. There is a need to reduce the friction between the abutting surfaces of the plug and expansion leaves or fingers below the friction between the external serrated outer surface of the expansion leaves or fingers and the bore hole wall.
U.S. Pat. No. 4,746,248 discloses an anchor bolt anchored by means of resin bonding. Hot-melt adhesive compositions are used as lubricants between the washer and the bolt head and are activated by heat as the bolt is rotated and tensioned in the bore hole. This patent does not disclose the use of mechanical expansion shells.
U.S. Pat. No. 4,619,559 discloses a friction reducing washer between a nut and the roof plate. The nut is threadedly secured to the end of a bolt extending from the bore hole and the friction reducing washer is utilized to increase the tension on the bolt.
U.S. Pat. No. 4,305,687 discloses the use of a roof anchor system consisting of a roof bolt with one or more wedge portions formed on its upper end. The anchor is secured in place by resin bonding material which is forced into contact with the walls of the hole by the wedge-shaped portion of the anchor. It is stated in the patent that compression is best achieved if the wedge-shaped portion of the bolt do not bond well to the grout. Therefore, coatings such as foil coatings, organic coatings or bonded lubricants such as teflon or molybdenum disulfide are placed on the wedge to allow it to move within the resin without being bound to the resin.